True, but in this context, they had a relational database, and the relational database is still there, and isn't going anywhere, so not a big win there.

Plus, the real problem is that the correct solution was something that was already boring at the time, memcached. If all you literally are using a "NoSQL" database like MongoDB for is a key-value store and literally nothing else, you don't need NoSQL, you just need a boring key-value store.

To operate? Do you mean for a developer to work with? Because most RDBMs systems I have worked with are much easier on the operator than most NoSQL systems for HA/DR.

No, not to work with a running system someone else operates, but to run it in production yourself.

Doesn't that rather depend on which SQL database you are using?

Indeed it does. There's a huge difference in ease of maintenance in production between (e.g.) PostgreSQL and MS SQL Server.